Controlling the transport and structural properties of oxide thin films through various parameters (such as temperature, strain, and electric field) makes them useful for technological applications including sensor, memory and logic devices. Recently, a mechanism was demonstrated for controlling the properties of a class of oxide materials, namely, gating them with ionic liquids. The voltage gating of an ionic liquid (IL) at the surface of an oxide film can create an electric field large enough that oxygen migrates from within the interior of the film to its surface, as illustrated in FIG. 1. This process is reversible and can be used with a large class of oxides having channels through which oxygen ions migrate. Such an ion transport channel is a collection of lattice sites along which oxygen ion diffusion occurs, as opposed to diffusion through one or more mesoscopic pores.
Of particular interest are the oxides VO2 and WO3, which can be reversibly gated for thicknesses at least as large as ˜120 nm. One consequence of the IL gating is the change in conductivity of the oxide films. In particular, for VO2, which displays a metal to insulator transition (MIT) near room temperature, one observes a suppression of this MIT even at low temperatures. The observed conductivity increase of the insulating state upon IL gating (application of positive gate voltage) is roughly three orders of magnitude and is non-volatile (see Jeong et al., “Suppression of metal-insulator transition in VO2 by electric-field induced oxygen vacancy formation”, Science, vol. 339, pp. 1402-1405, 2013). The material retains its conducting properties even when the bias voltage is reduced to zero and, further, even after the removal of the ionic liquid. The original high resistance state of the pristine material can be reached upon application of a reverse gate voltage (negative gate voltage). Upon IL gating (application of a positive gate voltage) in the case of WO3, which is a band insulator, one observes an increase in conductance of almost six orders of magnitude (see Altendorf et al., “Facet-independent electric-field-induced volume metallization of tungsten trioxide films”, Advanced Materials, 2016). For both of these oxides, the original insulating state is reached during reverse gating by migration of oxygen from the surface of the film and/or the ionic liquid to its interior. FIG. 2 shows an IL gating effect for a 10 nm VO2 thin film. The pristine film shows an MIT characteristic of VO2, which is suppressed by IL gating due to the creation of oxygen vacancies.
One observes a clear correlation between the increased conductivity of the film and the removal of oxygen from the film, while the converse is also true. In particular, the film conductivity depends on the oxygen present in the environment during the IL gating process. The presence of a sufficiently high concentration of oxygen can completely suppress any gate-induced conductivity increase (see FIG. 3). Other gases including nitrogen or argon have no significant effect on the IL gating process (see Li et al., “Suppression of ionic liquid gate-induced metallization of SrTiO3(001) by oxygen”, ACS Nano, vol. 13, pp. 4675-4678, 2013). This is evidence of an extremely high specificity of the IL gating process to oxygen.
The precise measurement of the oxygen concentration in a liquid medium is routinely necessary for applications in several industries such as medicine, biopharmaceutics, and the food and beverage industries. For example, the measurement of the amount of oxygen that is dissolved in blood (which is less than or equal to the oxygen saturation limit in blood) is critical for the diagnosis of several respiratory illnesses. The measurement of oxygen dissolved in a liquid requires an apparatus that separates oxygen from the sample being analyzed (known as the analyte) and then transports the separated oxygen to a measurement device. For example, in polarography (a widely used technique for the measurement of dissolved oxygen), oxygen from the analyte is transported through an oxygen-permeable membrane to an electrochemical cell. In the cell, reduction of oxygen results in a thermodynamically defined polarization voltage at a constant measurement current. The polarization voltage is directly correlated with the oxygen concentration and therefore the output voltage of the sensor is a measure of the oxygen in the analyte. However, these sensors are costly since platinum or gold electrodes are required as the working electrodes. Also, the requirement to build an electrochemical cell with reference electrodes for precise voltage measurement makes them bulky.